1. Field of the Invention
This invention relates to communication systems in general and, more particularly, to time division multiplexed serial loop communication systems in which data is transmitted in one direction around a loop transmission means.
2. Description of the Prior Art
Loop data communications systems have been known for several years and several control techniques have been devised in order to allow a master terminal or controller to communicate with a plurality of I/O terminals connected to the loop. One advantageously simple technique involves a form of time division multiplexing in which various time slots (also referred to herein as "channels") are permanently assigned to I/O terminals. Messages to and from I/O terminals are transmitted in the permanently assigned channel. It is apparent that such a system is relatively inefficient for interactive I/O terminals since the communications capability of a permanently assigned time slot or channel is wasted during periods of inactivity of the associated I/O terminal.
To allow terminals to use time slots not being used by other terminals, addressed message communications systems have been developed wherein data in each slot is preceded by the address of the receiving or sending terminal. These systems are also inefficient because band width is consumed in transmission of addresses which would otherwise be available for data transmission. Furthermore, contention problems arise when two or more terminals attempt to transmit at the same time. Contention has been resolved in the prior art by having the master terminal poll each of the I/O terminals for messages or by hub polling wherein each I/O terminal, after being polled, polls the next I/O terminal. These polling procedures likewise reduce band width available for data transmission and further complicate the communication system.
One prior art attempt to permit one terminal to utilize slots which have been assigned to another terminal is described in S/N 482,940 filed June 25, 1974 for Semistatic Time Division Multiplex Slot Assignment, by C. M. McClearn, Jr., et al. In McClearn, et al, each terminal operating on the loop must be assigned to a different base slot. A psuedo device latch is provided which may be set by a Write Echo command addressed to a non-existent device type and communicated in the slot assigned to that terminal. This pseudo device latch is reset by an End Operation command addressed to the same non-existent psuedo device to which the Write Echo command was originally directed. With this protocol, it is not possible to attach a plurality of terminals to the loop so as to communicate in the same base slot, and a multiple command procedure is required to disable one terminal from operating in its own slot, and establish communication with a second terminal in that slot. In order to accomplish this, for example to permit terminal C to obtain and then reliquish slots assigned to terminals A and B, would require that the controller perform the following steps: Send separate Write Echo commands in slots assigned to terminals A and B and addressed to non-existent devices. Then, send a Set Modulus command to terminal C, so that terminal C will operate in A and B's slots. After processing the operation involving communication with terminal C, the controller must then send a Set Modulus command to C to reduce its modulus, thereby leaving A and B's slots free. Finally, End Operation commands must be sent by the controller addressed to the same non-existent devices as above in slots pertaining to both A and B.
Thus, in McClearn, at least six different commands must be communicated by the controller in at least three different slots in order for one terminal to utilize slots pertaining to just two other terminals. This protocol is sufficiently complex that its implementation in a practical system is very difficult, and can be inefficient in utilization of communication band width under some circumstances.